States rights never trump the constitution. Not allowing gay marriage is purposefully discriminating against a particular (and significant) segment of society. How is that okay?
Where in the constitution does it say that one has a fundamental right to marry the same sex? If homosexuals were prohibited from marrying people of opposite sex, then I would buy your argument. But they are not, and they can--and many do--enter into traditional marriages. And many even have kids with their opposite sex partner. So in short, homosexuals have just as much right to a traditional marriage as anyone.
Edit: at least, that's my opinion until the Supreme Court rules otherwise, which they may or may not do. But as yet they have NEVER proclaimed there to be a fundamental right to marry people of the same sex.